Previous studies have indicated a high incidence of nonanemic iron deficiency in high school runners, but little is known regarding participants in other sports. The hematologic and iron status of 30 high school swimmers was assessed at the beginning and end of a competitive swim season. Iron depletion (serum ferritin level less than 12 micrograms/L) was initially present in 46.7% of the girls and none of the boys, yet among those subjects with hypoferritinemia there was no evidence of impaired red blood cell production. Serum haptoglobin levels were normal and did not correlate with serum ferritin levels. No significant change was observed in ferritin, hemoglobin, or haptoglobin concentrations during the course of the season. Dietary intake of iron was poor, particularly in the girls, in whom it averaged 43% of the recommended dietary allowance. Menstrual histories suggested an inverse relationship between the amount of menstrual flow and the serum ferritin level. These findings indicate a high prevalence of nonanemic iron deficiency in female high school swimmers that bears no relationship to swim training. Dietary inadequacies and menstrual blood flow appear to be important contributors to the hypoferritinemia observed in these athletes.